A threaded joint has been popularly used for connecting steel pipes used in an oil-producing industrial facility such as oil country tubular goods. In connecting steel pipes used for search and production of oil or gas, conventionally, a standard threaded joint which is stipulated in API (American Petroleum Institute) has been typically used. However, recently, deepening of a well for crude oil or a natural gas has progressed and the number of horizontal wells and inclined wells instead of vertical wells has been increasing and hence, the drilling and production environment has become difficult. Further, the number of wells developed in an extremely challenging environment such as oceans and polar regions has been increasing and hence, the performances which threaded joints have to achieve are diversified including compression resistance, bending resistance, and sealability against external pressure (external pressure resistance). In view of the above, the number of cases where a special threaded joint having high performances which is referred to as “premium joint” is being used has increased, and a demand for the improvement of performances of the special threaded joint has been also increased more and more.
A premium joint is a coupling-type joint where externally-threaded members (hereinafter referred to as “pins”) each of which includes a tapered thread, a seal portion (to be more specific, a metal to metal seal portion) and a shoulder portion (to be more specific, a torque shoulder portion) and is formed on a pipe end portion, and a internally-threaded member which includes tapered threads, seal portions (to be more specific, metal to metal seal portions) and shoulder portions (to be more specific, torque shoulder portions) and connects the pins to each other (hereinafter referred to as “box”) are jointed to each other. The tapered threads are important for firmly fixing the pipe joint, the seal portions play a role of ensuring air tightness by bringing the box and the pins into metal contact at such portions, and the shoulder portions forma shoulder surfaces which play a role of stoppers during fastening of the joint.
FIG. 4 is schematic explanatory views showing a conventional example of a premium joint for an oil country tubular goods use. FIG. 4 is also longitudinal cross-sectional views of a threaded joint of a circular pipe (a cross-sectional view where a pipe axis extends in the cross section). The threaded joint includes pins 3 and a box 1 corresponding to the pins 3. On an outer surface thereof, the pin 3 has an externally-threaded portion 7 and a nose portion (also referred to as a pin nose) 8 which is an elongated portion formed adjacent to the externally-threaded portion 7 and has no threads on a distal side of the pin 3. The nose portion 8 has a seal portion (to be more specific, a metal to metal seal portion) 11 on an outer peripheral surface thereof, and a shoulder portion 12 on an end surface thereof. On an inner surface thereof, the box 1 corresponding to the pins 3 has internally-threaded portions 5, seal portions 13 and shoulder portions 14 which are portions being threadedly engaged with or brought into contact with the externally threaded portions 7, the seal portions 11 and the shoulder portions 12 of the pins 3 respectively.
With respect to respective thread ridges formed on the pins 3 and respective thread roots formed on the box 1 corresponding to the thread ridges formed on the pins 3, a face on an opposite side to a pin pipe end (box end side) is referred to as a load flank face 30 (P being affixed to the load flank face 30 on a pin side and B being affixed to the load flank face 30 on a box side), and a face on a pin pipe end side (a box center side) is referred to as a stabbing flank face 31 (P being affixed to the stabbing flank face 31 on a pin side and B being affixed to the stabbing flank face 31 on a box side).
In the conventional example shown in FIG. 4, the seal portion 11 is formed on a distal portion of the pin 3, and desired seal performance can be realized by imparting an appropriate fastening torque. However, the fastening torque is influenced by a lubrication condition, a surface property and the like. As a design of a threaded joint which does not largely depend on these factors, there has been known a radial-directional seal method (also referred to as a radial seal type) where a radial directional component of a seal contact pressure is relatively increased.
With respect to the point where the seal portion is provided at a portion different from the shoulder portion, the radial-seal-type threaded joint is substantially equal to the conventional threaded joint having the seal portion on the distal portion of the pin.
The threaded portions of the threaded joint are not uniformly threadedly engaged with each other so that a concentrated load is generated at a thread end portion in general, and galling occurs when the concentrated load is excessively large.
As a prior art relating to this point, in patent literature 1, there is the description where a concentrated load is generated on stabbing flank faces of both end portions of a screw, and a countermeasure to cope with galling at the end portions of the box is particularly important. Accordingly, in the above-mentioned prior art, a concentrated load is decreased in such a manner that the stabbing flank faces 31 are formed by cutting with a pitch slightly increased only in a last pass or in several last passes in thread forming while keeping a pitch of load flank faces 30 at a fixed value so that a gap is formed between both stabbing flank faces 31B and 31P.